In my project, I take the UV-visible spectrum of reaction between pyridinium chlorochromate and N-acetylcysteine. In some wavelengths, the absorbance value is negative for example it is (-0.006, -0.002, -0.003){negative but very small}.

Also when I take the spectrum of pyridinium chlorochromate alone, there are absorbance values in negative {negative but very small}.

The blank is NaClO4 solution.

what are the reasons for these negative values? And can I negligible these values?

Thank you...

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    $\begingroup$ As they say in Russian, If you see a "buffalo" sign on an elephant's cage, do not trust your eyes. Same thing here. These values can't be. They are experimental errors. We may muse about the reasons, but the most important reason is that all experiments have errors. Bear with it. $\endgroup$ Feb 20, 2019 at 20:05
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    $\begingroup$ Could be the sample (the absorption of soluted pyridinium chlorochormate against pure water different to the one of soluted NaClO4 against pure water) in your region of interest. Could be instrument's design (two beam / simultaneous vs. consecutive measurement on single beam), (in)stability of the light source (e.g., age of the lamps), integration time (nm/min sweep rate), cell (glas/polymer). Balance the intensity of bands of interest vs. random fluctuations each recording has, e.g. by varying sample concentrations / cell length or average multiple spectra "recorded" (rather than "taken"). $\endgroup$
    – Buttonwood
    Feb 20, 2019 at 22:08

1 Answer 1


The negative values can arise due to two major reasons: You did not mention that whether you are using a single beam instrument or a double beam instrument. In a double beam instrument, absorbance of the sample and the blank is continuously measured. In a single beam spectrophotometer, A(blank) is recorded first and later subtracted. In each case, the output value at each wavelength is: Abs (measured) = A(sample)- A (blank)

If at any region of the spectrum A(blank) is larger you get slightly negative absorbance values. This could be due to several reasons including refractive index differences. Under ideal cases, the cuvet which contains the blank is same as cuvet containing the sample or both of them should be optically matched i.e. they should show identical absorbance readings at given wavelengths.

Another reason to see random negative values is that noise is a part of any instrumental analysis. Eliminating all sources of noises, you still get stuck with shot noise, which originates due to statistical counting of photons. That is the very nature of detection especially in spectroscopic measurements- perhaps the ultimate limit in analytical sciences.


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